Resource management - Rule for resource vs date clashing

Hi,

I'm looking to implement a rule so that when a specific resource is selected for an assignment, a check is done through the existing data to see whether they are already 'booked' for work within that calendar period.

In this example, you will see I have highlighted a row whereby a form of alert would be needed as John is already working on these days (row 1) - could this be done through conditional formatting foe example? Or are there any other options?

Also - ideally I'm looking for a solution which would enable to search data for date clashes over multiple Excel sheets. Really appreciate your help.

Many thanks, Greg

Please see attached file.

Program Managament.xlsx